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After reading most of the posts here I finally pulled the trigger and ordered a Modine Propane Hot Dawg 45,000 BTU heater for my garage. I installed it in my 600 sq ft garage (located in Northern Ohio on Lake Erie) yesterday. Today I went to Amerigas to obtain 2 40# propane tanks and a valve to switch from one tank to another as the working tank empties. I was told by both a tech and sales person that the setup I am asking for won't work due to the "working" tank freezing up before it is empty. It was explained to me that this is caused by the demand of the heater on the dwindling supply of propane in the working tank. It was suggested I get two 100# tanks (which I can't handle as they are about 175 pounds filled) or run the two 40# tanks in tandem - which to my mind defeats the purpose of using two tanks with a switch valve.

Can anyone shed some light on what to do next? I thought this setup was suppose to work.

I've never heard of that before, but if the people at the propane place told you that will happen...I'd suggest they likely know what they are talking about.

IIWY I'd go with the 2 100# tanks. Get a dolly or get some help moving them. That should be what...maybe 30 gallons of capacity? That still only 68 hours of "burner on time" for you heater. On an extremely cold day, in theory, you could run 2 40# tanks empty in a little more than a day.

A friend built a hog roaster and it had such a high usage rate that the 20lb tanks would freeze. I don't know about the demands of the furnace. I called the gas company and they brought a 250 gallon tank out. They have a 100 gallon also that can be put right next to the building.

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"If the women don't find you handsome, they should at least find you handy."

Propane is a liquid, but the heater burns vapor. (the spave above the liquid in the tank is vapor). It takes a certain amount of time for the liquid to vaporize. The cooler the temp, the longer it takes the liquid to vaporize. The larger tank is the more space for vapor to form....

So if the heater is running for long times when it's really cold, you will run out of vapor and it will FREEZE up.

not sure if i should get in to this, but propane tanks dont freeze on the inside, they will grow frost on the out side of the tank, which is caused by a change of state of the propane inside the tank, propane and natural gas have less than 1/2% moisture.

the frost grows as the pressure in the tank falls and the tank becomes cold in the area where the propane is changing from a liquid to a gas, which is why one dosent normally see that when using a new tank.

your tank is too small.
propane liquid in the tank is -44 degrees and boiling the vapor is what is coming out of the tank and buring, if the rate of evap is not enough for the demand of the burner the tank will frost over and the burner will at some point stop no matter if the tank is near empty or not. The guy at Amerigas is right, you need to run both the 40s or get a 100lbs.

When the propane man talks about "freeze up" what he means is the temperature of the Propane in the tank drops close enough to the boiling point that it can no longer supply sufficient vapor to meet the withdrawl demands.

The rate of vaporization is dependent on the outside air temperature and wetted surface area of the tank.

Vaporization rate chart for a 100lb cylinder:

Source: Rego's LP-Gas Serviceman's Manual

You'll notice that even a 100lb cylinder falls a little short on capacity when the temperature is below 20F and the tank is nearly empty. Your heater might stop with a little bit of fuel left in the tank.

If you plan to use this heater often, I would see about having a small propane tank delivered. Tank rental is generally free if you buy a certain amount of gas. Even if you have to pay the rental fee, it's really minimal.

Fat,
Why not just get them to bring a tank and get set up on automatic delivery ? Thats what we had for almost 10 years at my last shop and it worked out great. There was no charge for the tank and no hassles having to go get anything

the 100 lb tank is so you will still have some propane when it freezes, try putting that electric heat cord for pipes around the tank, it works but it costs you power, best deal is to get a bigger tank, in the winter 100 pounders pull real easy when on there side in the snow

I run a Mr Heater Big Maxx 45,000 propane heater that I installed last year and I use 2-40lb bottles with a automatic switch over valve hooked up to them. When 1 bottle empties, it switches over to the full one and flags the empty one. Last winter during the really cold snaps we had in Lorain county Ohio, I would run through a bottle in about a week and half, otherwise it would last 2-3 weeks and that is with the t-stat set at 50* and leaving it to turn itself on and off when needed. I didn't experience any problems with my bottles and they always ran to empty. They would frost up some near the end but the heater doesn't run alot. I used the 40lb bottles for ease of handling and transporting in my truck. My garage is well insulated which helps in keeping the heater from running much unless I go out to work on something but otherwise not bad.